This invention relates to a method for automatic control of sensitivity for an implanted cardiac control device, such as a pacemaker, cardioversion and/or cardiac defibrillation device, and to corresponding devices having enhanced sensitivity control.
In cardiac control devices, such as pacemakers and defibrillators, the pertinent electrical signals have amplitudes spread over a range extending, e.g., from a few tenths of a millivolt (e.g., the amplitude of the P wave is in the region of 0.4 mV) to a few millivolts (e.g., the amplitude of the R wave is often between 5 and 10 mV). To ensure satisfactory operation whatever the amplitude of the signal received, some of these cardiac control devices are fitted with an automatic gain control.
French patent No. 2,606,644 describes an implantable device having a pacing and cardioversion function, using an automatic gain control in the amplitude circuit of weak cardiac signals, such as those of ventricular fibrillation.
European patent No. 340,045 describes a cardioversion device using three sensitivities: the first being an average sensitivity for detection of the sinus rhythm and ventricular tachycardia, the second being a higher sensitivity for differentiating ventricular fibrillation and asystole, and the third being a lower sensitivity for differentiating the R wave and the lesional T wave at high amplitude. However, one of these three sensitivities is selected as a function of the status of the device (period of suspected tachycardia, or postshock period), and is maintained at least until the next cycle.
There are two types of drawbacks in the case of sensitivity that can be switched between three values. The first is that of oversensing when the sensitivity is too high: T waves can then be detected and mistaken for R waves upon which the pacemaker recycles itself. This can lead the rhythm analysis system to err (e.g., the rhythm is found to be faster than it actually is). The second is that of undersensing when the sensitivity is too low. In this case, R waves can fail to be detected and the operating of the device is no longer adapted to the patient's heart.
U.S. Pat. No. 5,117,824 refers to a cardiac pacemaker and/or cardioverter/defibrillator that has an R-wave detector and adjusts its sensing threshold in response to R-wave amplitude. The detection threshold is selected as a predetermined proportion of the amplitude of a detected R-wave. The threshold then decays over a period of less than three seconds to a fixed, lower threshold value. Redefinition of the threshold is prevented for a predetermined time period following delivery of a pacing pulse to return the threshold to its lowest value during pacing.